(1) Field of the Invention
This invention relates to de-focussing the electrode pattern image used to selectively expose a layer of photoresist in order to form an electrode pattern mask. The electrode pattern mask results in electrodes having a trapezoidal cross section and keyholes are avoided.
(2) Description of the Related Art
In the manufacture of integrated circuit wafers photolithographic steps are frequently used to transfer an electrode pattern comprising a number of closely spaced electrodes from an electrode pattern mask to a metal conductor layer using a photoresist mask. To form the photoresist mask the electrode pattern must be focussed on a layer of photoresist to selectively expose the layer of photoresist. After the electrodes have been formed conformal dielectric layer is formed over the electrodes. Restricted regions, or keyholes, often form in the dielectric layer causing problems in later processing steps.
U.S. Pat. No. 5,496,669 to Pforr et al. describes a system comprising a latent image detection device which can detect the coincidence of mask alignment features. A focus test pattern on the product wafer is used to allow measurement of substrate height and focus.
U.S. Pat. No. 5,447,810 to Chen et al. describes a mask and method of using off-axis illumination to provide increased depth of focus and minimize critical dimension differences between certain features.
U.S. Pat. No. 5,362,585 to Adams describes a method to achieve good stepper focus and exposure over an entire wafer for a particular wafer before a product run is started.
U.S. Pat. No. 5,439,767 to Yamashita et al. describes a method of measuring the transmittance error and the phase shift angle error of a phase shift mask.
TSMC-96-181, Ser. No. 08/803,352, Filed Feb. 20, 1997, Entitled "A System for In-Line Monitoring of Photo Processing De-Focus and Image Tilt in VLSI Fabrication," assigned to the same assignee, describes a de-focus test pattern and a method for in-line monitoring of photo processing steps in the fabrication of integrated circuit wafers.
This invention describes a method of intentionally de-focussing the image of an electrode pattern mask used to selectively expose a layer of photoresist. The resulting electrode pattern has a trapezoidal cross section. A conformal dielectric coating is then used to passivate the electrode pattern. Restricted regions and keyholes in the dielectric coating are avoided.